canon = it's in the show. which it is clearly not
fanon = fans (as few as one fan as many as all of them) say/think this is how it is

I said it wasn't /normal/ fanon, being that i've been a brony since mid season 1 and never heard it untill now.
Being a fan, you can think whatever you want about any character's back story or whatever (as long as it hasn't been clearly contradicted in the show somehow. which this statement about Fluttershy being molested has not been so feel free to think it)